In accordance with an enhanced interest in natural renewable energy, a solar cell using solar light power generation is attracting people's attention. In particular, development of a Si-based solar cell using a silicon (hereafter referred to as “Si”) based material that is abundant in the amount of resources and capable of saving the resources and reducing the costs is eagerly carried out.
In this type of Si-based solar cell, a p-type semiconductor formed of a p-type Si-based material and an n-type semiconductor formed of an n-type Si-based material are joined to form a p-n junction. Further, when a depletion layer formed at the interface of the p-n junction is irradiated with solar light, the carriers are excited to generate photoelectric current, and this is subjected to photoelectric conversion to be output to the outside.
However, regarding such a Si-based solar cell of p-n junction type, the energy conversion efficiency is theoretically as low as about 30% at the maximum and, for this reason, it is demanded that a solar cell having a larger energy conversion efficiency at a low cost is realized.
Thus, a Si quantum dot solar cell in which a Si quantum dot layer is interposed between the aforesaid p-type semiconductor and the aforesaid n-type semiconductor is recently attracting people's attention.
This Si quantum dot solar cell aims at effectively utilizing the solar light spectrum by applying a quantum dynamics theory to nanotechnology, and this enables a large improvement in the conversion efficiency, so that the Si quantum dot solar cell is regarded as being prospective as a next-generation solar cell.
With regard to this type of Si quantum dot solar cell, a method disclosed in non-patent document 1, for example, is known as a method for fabricating a quantum dot structure.
This non-patent document 1 provides reports on a residual stress of Si nanocrystals dispersed in a SiO2 matrix, where an amorphous silicon (a-Si) layer and a silicon dioxide (SiO2) layer are alternately stacked and thermally treated at a temperature of about 1100° C. This allows that Si quantum dots are obtained in a mode in which Si of the a-Si layer is deposited and dispersed in the SiO2 matrix.
Also, non-patent document 2 provides reports on a Si quantum dot nanostructure for a tandem-type photovoltaic cell.
In this non-patent document 2, a SiO2 layer and a Si-rich oxide (SRO) layer are alternately stacked and thermally treated at a temperature of 1050° C. to 1150° C. in a nitrogen atmosphere. This allows that, in the same manner as in non-patent document 1, Si quantum dots are obtained in a mode in which Si in the SRO layer is deposited and dispersed in the SiO2 matrix.
Also, this non-patent document 2 reports that a silicon nitride (Si3Ni4) layer or a silicon carbide (SiC) layer can be used instead of the SiO2 layer.    Non-patent Document 1: T. Arguirov et al., “Residual stress in Si nanocrystals embedded in a SiO2 matrix”, Applied Physics Letters, 89, 053111 (2006) pp. 6748-6756    Non-patent Document 2: G. Conibeer et. al., “Silicon quantum dot nanostructures for tandem photovoltaic cells”, Thin Solid Films, 516 (2008) pp. 6748-6756